1. Technical Field
The invention generally relates to organic fuel cells and in particular liquid feed organic fuel cells.
2. Background Art
Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. In an organic/air fuel cell, an organic fuel such as methanol, formaldehyde, or formic acid is oxidized to carbon dioxide at an anode, while air or oxygen is reduced to water at a cathode. Fuel cells employing organic fuels are extremely attractive for both stationary and portable applications, in part, because of the high specific energy of the organic fuels, e.g., the specific energy of methanol is 6232 Wh/kg.
Two types of organic/air fuel cells are generally known:
1. An "indirect" or "reformer" fuel cell in which the organic fuel is catalytically reformed and processed into carbon monoxide-free hydrogen, with the hydrogen so obtained oxidized at the anode of the fuel cell.
2. A "direct oxidation" fuel cell in which the organic fuel is directly fed into the fuel cell without any previous chemical modification where the fuel is oxidized at the anode.
Direct oxidation fuel cells do not require a fuel processing stage. Hence, direct oxidation fuel cells offer a considerable weight and volume advantage over the indirect fuel cells. Direct oxidation fuel cells use either a vapor or a liquid feed of the organic fuel. Current art direct oxidation fuel cells that have shown promise typically employ a liquid feed design in which a liquid mixture of the organic fuel and a sulfuric acid electrolyte is circulated past the anode of the fuel cell.
The use of sulfuric acid electrolyte in the current-art direct methanol fuel cells presents several problems. The use of sulfuric acid, which is highly corrosive, places significant constraints on the materials of construction of the fuel cell. Typically, expensive corrosion resistant materials are required. Sulfate anions, created within the fuel cell, have a strong tendency to adsorb on the electrocatalyst, thereby hindering the kinetics of electro-oxidation of the fuel and resulting in poor performance of the fuel electrode. Also, sulfuric acid tends to degrade at temperatures greater than 80.degree. C. and the products of degradation usually contain sulfur which can poison the electrocatalyst. In multi-cell stacks, the use of sulfuric acid electrolyte can result in parasitic shunt currents.
Exemplary fuel cells of both the direct and indirect types are described in U.S. Pat. Nos.: 3,013,908; 3,113,049; 4,262,063; 4,407,905; 4,390,603; 4,612,261; 4,478,917; 4,537,840; 4,562,123; and 4,629,664.
U.S. Pat. Nos. 3,013,908 and 3,113,049, for example, describe liquid feed direct methanol fuel cells using a sulfuric acid electrolyte. U.S. Pat. Nos. 4,262,063, 4,390,603, 4,478,917 and 4,629,664 describe improvements to sulfuric acid-based methanol fuel cells wherein a high molecular weight electrolyte or a solid proton conducting membrane is interposed between the cathode and the anode as an ionically conducting layer to reduce crossover of the organic fuel from the anode to the cathode. Although the use of the ionically conducting layer helps reduce crossover, the ionically conducting layer is used only in conjunction with a sulfuric acid electrolyte. Hence, the fuel cell suffers from the various aforementioned disadvantages of using sulfuric acid as an electrolyte.
In view of the aforementioned problems associated with using sulfuric acid as an electrolyte, it would be desirable to provide a liquid feed fuel cell that does not require sulfuric acid as an electrolyte.
In addition to the improvements in operational characteristics of the liquid feed fuel cell, the conventional method of fabricating high-surface-area electro-catalytic electrodes for use such fuel cells also needs to be improved. The existing method of fabrication of fuel cell electrodes is a fairly time-consuming and expensive procedure. Specifically, electrode fabrication requires that a high surface-area carbon-supported alloy powder be initially prepared by a chemical method which usually requires about 24 hours. Once prepared, the carbon-supported alloy powder is combined with a Teflon.TM. binder and applied to a carbon fiber-based support to yield a gas diffusion electrode. To volatilize impurities arising out of the Teflon.TM. binder and to obtain a fibrous matrix of Teflon.TM., the electrodes are heated to 200.degree.-300.degree. C. During this heating step, oxidation and sintering of the electrocatalyst can occur, resulting in a reduced activity of the surface of the electrode. Thus, the electrodes often require re-activation before use.
Also electrodes produced by conventional methods are usually of the gas-diffusion type and cannot be effectively used in liquid feed type fuel cells as the electrode is not adequately wetted by the liquid fuel. In general, the structure and properties of a fuel oxidation electrode (anode) for use in liquid feed type fuel cells are quite different from the gas/vapor feed fuel cells such as the hydrogen/oxygen fuel cell. The electrode structures for use in a liquid feed fuel cell should be very porous and the liquid fuel solution should wet all pores. Carbon dioxide that is evolved at the fuel electrode should be effectively released from the zone of reaction. Adequate wetting of the electrodes is a major problem for liquid feed fuel cells--even for those which use a sulfuric acid electrolyte.
As can be appreciated, it would be desirable to provide improved methods for fabricating electrodes, particularly for use in liquid feed fuel cells. It is also desirable to devise methods for modifying electrodes, originally adapted for gas-feed fuel cells, for use in liquid feed fuel cells.
In addition to improving the liquid feed fuel cell itself and for providing improved methods for fabricating the electrodes of fuel cell, it would be desirable to provide new effective fuels as well. In general, it is desirable to provide liquid fuels which undergo clean and efficient electro-chemical oxidation within the fuel cell. The efficient utilization of organic fuels in direct oxidation fuel cells is, in general, governed by the ease by which the organic compounds are anodically oxidized within the fuel cell. Conventional organic fuels, such as methanol, present considerable difficulties with respect to electro-oxidation. In particular, the electro-oxidation of organic compounds such as methanol involves multiple electron transfer and is a very hindered process with several intermediate steps. These steps involve dissociative adsorption of the fuel molecule to form active surface species which undergo relatively facile oxidation. The ease of dissociative adsorption and surface reaction usually determines the facility of electro-oxidation. Other conventional fuels, such as formaldehyde, are more easily oxidized, but have other disadvantages as well. For example, formaldehyde is highly toxic. Also, formaldehyde is extremely soluble in water and therefore crosses over to the cathode of the fuel cell, thus reducing the performance of the fuel cell. Other conventional organic fuels, such as formic acid, are corrosive. Furthermore, many of the conventional organic fuels poison the electrodes of the fuel cell during electro-oxidation, thus preventing sustained operation. As can be appreciated, it would be desirable to provide improved fuels, particularly for use in liquid feed fuel cells, which overcome the disadvantages of conventional organic fuels, such as methanol, formaldehyde, and formic acid.